1. Field of the Invention
This invention relates generally to a device for testing magnetic read/write heads and more specifically to such a device in which there is a rotating magnetic disk on which servo information has been written.
2. Description of the Related Art
Before their installation in a conventional direct access storage device (DASD), magnetic read/write heads are first tested in a device specifically designed for the testing process. A magnetic head tester is basically a rudimentary disk drive that includes a drive motor and spindle (called a spin stand), at least one magnetic disk mounted on the drive motor spindle and capable of being rotated thereby, an actuator on which the head to be tested is mounted and an electromechanical system, called a servomechanism (or, equivalently, a servo-control mechanism), for accurately positioning the actuator (and its mounted head) relative to the rotating disk. The testing device also includes a micro-positioner and associated circuitry for writing servo-tracks, to be discussed below, on the magnetic disk, so that the head can be accurately positioned at target locations on the disk.
In order to test the read and/or write capabilities of a head, it is necessary to accurately position the head at various places on a disk mounted within the tester. To insure the accuracy of this positioning process, the disk mounted within the tester is furnished with embedded information, called servo information, which is data stored (as “bursts” of magnetic transitions) within sectors of small angular width that are periodically distributed along radially narrow, concentric, annular circular tracks on the disk. This servo information, which, in the prior art, is written on the disk while the disk is already mounted in the tester, is written using a micro-positioner affixed within the tester and the head already mounted in the tester. The micro-positioner is a device that can incrementally move the head to proper positions at which to write the servo information.
The servo information identifies radial positions within the annular width of the tracks, so that the track center can be accurately located and it also identifies the angular positions of data-free sectors within the tracks, typically adjacent to the sectors containing the servo information, on which data can subsequently be written and read by the head during the testing process. The role of the servo data is of great importance in accurately positioning the head relative to the center of the track and in maintaining that position during portions of the testing process. Thus, servo data supplies both track identifying information and positioning information
When the actuator mounted head seeks a particular position on the disk (the target position), which is typically the radial center of a track at some angular position along the track, the servo information located adjacent to that position is read by the head being tested and used to determine whether the head is actually located where it is supposed to be. This servo information is read immediately before the head reaches the target position and is transmitted to what is called servo-loop circuitry that is an integral part of the servomechanism of the tester. The difference (if it exists) between the intended location of the actuator (the target) and the actual location of the actuator as indicated by the adjacent servo data, generates a position error signal (PES) that is then used to correct the actuator positioning on the track. The PES, which is digital data, is supplied to a digital-to-analog converter (DAC) that generates a current proportional to the digital PES and, in turn, activates a voice coil motor (VCM), which is a current carrying coil positioned between permanent magnets. The VCM then responds to the DAC generated current and repositions the actuator.
There are many reasons why the actuator may be positioned incorrectly. Random vibrations can easily move the head slightly from its target, but much of the position error is related to the fact that the typical magnetic disk is slightly warped or generally fails to rotate perfectly on its spindle. Thus, even if the actuator is locked at what should be the fixed radial position of the target track's center line, the projection of the head on the rotating disk is not a circle concentric with the disk center, whereupon the head fails to follow the target track's center line and may, in fact, overlap several tracks.
The servo data, because of the process with which it is written, is presumably located on tracks that are essentially concentric circles of small radial width (circular annuli). Therefore, when the disk rotates improperly, there is a discrepancy between the track followed by the actuator (and mounted head) and a circular track produced by the servo data writing process. Nevertheless, the embedded servo data is supposed to rectify the improper tracking by means of the provided PES.
Writing accurate servo data on the disk is an important, expensive and time consuming task that is done within the head tester itself using an open-loop or closed-loop micro-positioner and the tester's head.
The additional elements within the tester needed to write servo data, including the micro-positioner and its associated circuitry, add greatly to the expense of the tester. In addition, the use of these additional elements, as noted, adds to the time required to complete the testing process. Even with the added time and expense, however, the repeatability of the servo pattern on a plurality of disks is often questionable and it is difficult to verify pattern accuracy. This is because the head testing device is not an optimal device within which to write servo data.
Takano et al. (US Patent Application Publication: US 2002/0018314 A1) describes a magnetic-disk evaluation apparatus in which servo information is both written on a disk and tested for its accuracy. In such a disk-evaluation process, servo information is first written on the disk in a temporary form, often by simply copying it from another disk. If this servo information is of sufficient accuracy, the disk can be tested for compliance with certain industry standards and, if those standards are met, the disk is then mounted within its final hard disk drive where more accurate servo information is written upon it. A problem arises if the temporary information is of insufficient accuracy to permit the disk to be mounted for writing of the permanent information. Takano et al. therefore provide a system in which the temporary servo signals can be evaluated using a measuring device and positioner to determine whether the arm of the evaluation apparatus is displaced from a target position by an unacceptable amount.
The present inventors, therefore, proposed the use of a disk in a head testing apparatus in which embedded servo data has been pre-written outside the testing apparatus within a dedicated servo disk writer. This invention constitutes related patent application, Ser. No. 11/217,872, that is hereby incorporated in its entirety by reference.
The sole task of a dedicated servo disk writer is to efficiently, accurately and repeatedly write servo information on disks. Such a device can do the job with much greater accuracy and repeatability than can be accomplished using a head tester with an included micro-positioner as a servo track writer. Moreover, by allocating the task of servo track writing to a device that is designed specifically to perform such a task, the necessity of using the head tester to perform the same task in a less exact, repeatable and efficient manner, is eliminated. Thus, the head tester can be simplified and dedicated to doing what it is meant to do, namely to test heads.
A problem arises, however, when a disk that has its servo tracks written in one apparatus is then transferred to another. The problem is a result of the fact that although servo data is written on circular tracks that are substantially concentric when originally written on the disk, these tracks will generally be eccentric when the disk is rotated by the drive spindle of the apparatus to which it is transferred (the host apparatus). This, of course, will be the inevitable result if the drive spindle of the original servo writer has even the slightest wobble or if the disk itself slips, is warped or off-center, or if the writing apparatus was subjected to thermal or mechanical shocks during the writing process or if the host apparatus has similar problems. Given that present 3.5″ disks have a track pitch of approximately 100,000 tracks per inch (TPI), it can be seen that the radial width of each track is miniscule and that the slightest variations in track concentricity will be exacerbated by variability between machines or external perturbations. Thus, if a dedicated servo writer is to be used to write the servo information on a disk that is then mounted in a head tester, the head tester must have the ability to either eliminate or compensate for the eccentricities of the pre-written servo data.
The lack of track concentricity and other written-in positioning errors encountered by a head testing device attempting to read what were supposed to be concentric circular tracks when they were originally written, become what are called repeatable position errors. Even if the original tracks were perfectly concentric circles, but their concentricity is lost in the host tester, then new writes by the read/write head, even if the head is kept at a substantially fixed radial position, can cross over several tracks and can overwrite previously written data. This phenomenon is called repeatable runout (RRO). It is to be noted that the RRO problem is not restricted to head testing devices, but is also a common problem in the disk drives of actual DASD's themselves. Some key sources of repeatable position errors (including RRO) in following a servo track are the following:                1) RRO due to shift in center of rotation, before and after servo-track write (already discussed above).        2) Uneven track spacing written by the servo track writer.        3) Repeatable error due to position error written-in by the servo track writer.        4) Vibration induced RRO.        5) “Apparent” position error due to media noise, media variations and defects.        
Wherever such errors occur, accurate positioning of the read/write head becomes nearly impossible without additional information being present to guide the head to the correct track positions and, when possible, to correct for RRO in some systematic way. This additional information, which is then used to actuate the locating and position-correcting servo-mechanisms within the host drive unit, is the servo information already contained within the tracks themselves.
Along with the RRO type of position error that is associated with off-center drive spindles, as in 1) above, or warped disks, there is the more difficult problem of position errors associated with random mechanical, electrical and thermal perturbations of the drive system and/or disk as included under 4) and 5) above. When such random errors in the pre-written disk are carried over onto a host drive, they show up as repeatable errors. In turn, these repeatable errors cause the servomechanism of the host drive to constantly respond, in an effort to provide a position correction. The associated accelerations of the actuator causes undesirable effects such as uneven track spacing, track encroachment and inconsistent measurements.
In general, the servo data embedded in the disk provides enough information to fully characterize the effects of repeatable position errors and, therefore, it provides enough information to eliminate or significantly suppress those effects. The invention Ser. No. 11/217,872 cited above and incorporated herein by reference specifically addressed the problem of eliminating the effects of RRO in 1), namely RRO caused by an off-center spindle in a disk servo-written on a dedicated servo-track writer. It will be an object of the present invention to address the problem of eliminating other effects of RRO, such as those in 2-5, once the effects of drive spindle eccentricity RRO have been eliminated.
Prior art discloses various attempts to eliminate the effects of RRO. Melkote et al. (U.S. Pat. No. 6,999,267) disclose a method for determining the PES for each servo sector, to be used in iteratively determining proper compensation for RRO.
Ehrlich (US Published Patent Application 2005/0237645) addresses the misplacement of the head while writing. This problem is resolved by adjusting the width of the servo burst in order to place the servo burst in a correct position to properly locate the head.
Hanson et al. (US Published Patent Application 2003/0112545) teaches computing a coherent RRO by averaging PES over multiple tracks so as to produce a zero acceleration profile.
Unlike the cited prior art, the present invention describes a method to measure and compute the complete written-in position error (errors 1-5 above) so as to provide a reference profile for subsequent track following operations that allows the error to be effectively eliminated as is made evident by a position error signal (PES) of essentially zero amplitude (see FIG. 4). The new track profile now being followed, with the effective elimination of the center-line eccentricities, will be called an “Iterative Repeatable Runout Nulling Profile” or an IRON Profile, since the actuator arm treats the center line as a null position and the head moves only relative to that position with nearly zero acceleration during track following operations.